1. Field of the Invention
The present invention relates to biodegradable polymer compositions, in particular those containing both phosphate and desaminotyrosyl L-tyrosine ester linkages in the polymer backbone and that degrade in vivo into non-toxic residues. The polymers of the invention are particularly useful as implantable medical devices and drug delivery systems.
2. Description of the Prior Art
Biocompatible polymeric materials have been used extensively in therapeutic drug delivery and medical implant device applications. Sometimes, it is also desirable for such polymers to be, not only biocompatible, but also biodegradable to obviate the need for removing the polymer once its therapeutic value has been exhausted.
Conventional methods of drug delivery, such as frequent periodic dosing, are not ideal in many cases. For example, with highly toxic drugs, frequent conventional dosing can result in high initial drug levels at the time of dosing, often at near-toxic levels, followed by low drug levels between doses that can be below the level of their therapeutic value. However, with controlled drug delivery, drug levels can be more nearly maintained at therapeutic, but non-toxic, levels by controlled release in a predictable manner over a longer term.
If a biodegradable medical device is intended for use as a drug delivery or other controlled-release system, using a polymeric carrier is one effective means to deliver the therapeutic agent locally and in a controlled fashion, see Langer et al., Rev. Macro. Chem. Phys., C23(1), 61 (1983). As a result, less total drug is required, and toxic side effects can be minimized. Polymers have been used as carriers of therapeutic agents to effect a localized and sustained release. See Chien et al., Novel Drug Delivery Systems (1982). Such delivery systems offer the potential of enhanced therapeutic efficacy and reduced overall toxicity.
For a non-biodegradable matrix, the steps leading to release of the therapeutic agent are water diffusion into the matrix, dissolution of the therapeutic agent, and diffusion of the therapeutic agent out through the channels of the matrix. As a consequence, the mean residence time of the therapeutic agent existing in the soluble state is longer for a non-biodegradable matrix than for a biodegradable matrix, for which passage through the channels of the matrix, while it may occur, is no longer required. Since many pharmaceuticals have short half-lives, therapeutic agents can decompose or become inactivated within the non-biodegradable matrix before they are released. This issue is particularly significant for many bio-macromolecules and smaller polypeptides, since these molecules are generally hydrolytically unstable and have low permeability through a polymer matrix. In fact, in a non-biodegradable matrix, many bio-macromolecules aggregate and precipitate, blocking the channels necessary for diffusion out of the carrier matrix.
These problems are alleviated by using a biodegradable matrix that, in addition to some diffusional release, also allows controlled release of the therapeutic agent by degradation of the polymer matrix. Examples of classes of synthetic polymers that have been studied as possible biodegradable materials include polyesters (Pitt et al., Controlled Release of Bioactive Materials, (Baker, ed. 1980); polyamides; polyurethanes; polyorthoesters (Heller et al., Polymer Engineering Sci., 21:727 (1981); and polyanhydrides (Leong et al., Biomaterials 7:364 (1986). Specific examples of biodegradable materials that are used as medical implant materials are polylactide, polyglycolide, polydioxanone, poly(lactide-co-glycolide), poly(glycolide-co-polydioxanone), polyanhydrides, poly(glycolide-co-trimethylene carbonate), and poly(glycolide-co-caprolactone).
Polymers having phosphate linkages, called poly(phosphates), poly(phosphonates) and poly(phosphites), are known. The respective structures of these three classes of compounds, each having a different sidechain connected to the phosphorus atom, are as follows: ##STR2##
The versatility of these polymers comes from the versatility of the phosphorus atom, which is known for a multiplicity of reactions. Its bonding can involve the 3p orbitals or various 3s-3p hybrids; spd hybrids are also possible because of the accessible d orbitals. Thus, the physico-chemical properties of the poly(phosphoesters) can be readily changed by varying either the R or R' group. The biodegradability of the polymer is due primarily to the physiologically labile phosphoester bond in the backbone of the polymer. By manipulating the backbone or the sidechain, a wide range of biodegradation rates are attainable.
An additional feature of poly(phosphoesters) is the availability of functional side groups. Because phosphorus can be pentavalent, drug molecules or other biologically active substances can be chemically linked to the polymer, as shown by Leong, U.S. Pat. Nos. 5,194,581 and 5,256,765. For example, drugs with --O--carboxy groups may be coupled to the phosphorus via an ester bond, which is hydrolyzable. The P--O--C group in the backbone also lowers the glass transition temperature of the polymer and, importantly, confers solubility in common organic solvents, which is desirable for easy characterization and processing.
Kohn et al., U.S. Pat. No. 4,638,045, discloses bioerodible polymers comprising monomer units of two or three amino acids polymerized via hydrolytically labile bonds at their respective side chains, rather than at the amino- or carboxylic acid-terminals by amide bonds. Zalipsky et al., U.S. Pat. No. 5,219,564, discloses copolymers of poly(alkylene oxides) and amino acids having pendent functional groups capable of being conjugated with pharmaceutically active compounds for drug delivery systems.
Kohn et al., U.S. Pat. No. 5,099,060, describes a particularly preferred monomer for making amino-acid derived poly(iminocarbonates) as: ##STR3## The resulting poly(iminocarbonate) type polymers are said to be hydrolytically unstable and yet exhibit improved thermal stability for convenient processing. Similar tyrosine-derived poly(carbonate) compounds have been reported as promising orthopedic implant materials. Ertel et al., "Evaluation of Poly(DTH Carbonate), a Tyrosine-derived Degradable Polymer, for Orthopedic Applications", J. of Biomed. Materials Res., 29:1337-48 (1995); and Choueka et al., "Canine Bone Response to Tyrosine-derived Polycarbonates and Poly(L-lactic Acid)", J. of Biomed. Materials Res., 31:35-41 (1996). However, there has been a need for materials to degreade at a significantly higher rate than desaminotyrosyl L-tyrosine based poly(iminocarbonates), and none of these documents suggests the use of phosphoester linkages in combination with amino acid-derived monomeric units for this purpose.